A conventional magnetic impedance device utilizes a magnetic impedance effect, and is disclosed in Japanese Patent Application Publication No. H08-75835. The magnetic impedance effect is that impedance of the device changes in accordance with an outside stress in a case where the device is energized with an alternating current (e.g., a high frequency alternating current, the frequency being higher than 1 MHz). The device includes a magnetic layer, which is made of amorphous alloy and has a soft magnetic property. Here, the amorphous alloy has high relative magnetic permeability. Therefore, a change of the magnetic permeability in the magnetic layer in accordance with an external magnetic field becomes large, so that the device has high sensitivity.
However, the magnetic impedance device with the magnetic layer made of amorphous alloy has low heat resistance, so that the sensitivity of the device is much decreased in a case where the device is processed with heat treatment above almost 400° C. The reason is as follows. The crystallization temperature of the magnetic layer made of amorphous alloy is low, i.e., at around 400° C. Therefore, when the device is processed with heat treatment above almost 400° C., the amorphous alloy is crystallized, so that the soft magnetic property of the amorphous alloy disappears. Here, the soft magnetic property of the amorphous alloy provides high sensitivity magnetic impedance.
Further, in a case where the magnetic layer is formed of easily oxidizable material, the magnetic layer is oxidized with heat treatment, so that the soft magnetic property is deteriorated. Thus, the sensitivity is decreased.
Therefore, it is difficult to manufacture the magnetic impedance device having the magnetic layer made of amorphous alloy with using a conventional semiconductor processing method. That is because the conventional method usually includes a step of heat treatment above almost 400° C. Accordingly, it is difficult to minimize the device with using the conventional method so that the device is integrated with another circuit such as a sensor output signal processor.
Further, when the device is annealed, i.e., processed with heat treatment, a stress is generated in a substrate since thermal expansion of the substrate is different from that of the device. Here, the device is mounted on the substrate. Therefore, in some cases, the device may be removed from the substrate. To prevent from being removed, deposition condition for depositing a magnetic layer composing a magnetic impedance device is changed, or a film quality of the magnetic layer is changed. This is disclosed in Japanese Patent Application Publication No. 2001-228229. However, this device is necessitated to form with limited manufacturing method and to have a limited construction.
Moreover, since a magnetic impedance device having high sensitivity is available for various sensor systems, minimization and low manufacturing cost are much required. For example, a magnetic impedance head module according to a prior art having a thin film magnetic impedance device is disclosed in Japanese Patent Application Publications No. 2001-318131. The head module includes the thin film magnetic impedance device, an electric power supply circuit for energizing the device with a high frequency alternating current, and a detection circuit for detecting a impedance change, which are provided with a discrete circuit. And each discrete circuit is combined with a hybrid IC. Therefore, minimization and reduction of manufacturing cost of the head module are limited.
Further, a magnetic impedance device is suitably used for a sensor apparatus mounted on an automotive vehicle, the sensor apparatus detecting, for example, rotation of a rotational body. A rotation sensor apparatus according to a prior art is disclosed in Japanese Patent Applications No. H08-304432 (i.e., U.S. Pat. No. 5,841,276) and No. 2000-46513. These sensor apparatuses are mounted on an engine of a vehicle or on a wheel hub, so that the sensor apparatuses detect rotation of crankshaft of the engine or rotation of wheel of the vehicle, respectively. In each case, it is required to minimize the sensor apparatus so as to improve mounting performance of the apparatus and to increase design freedom of an engine and so on.
Further, the magnetic impedance device mounted on the vehicle is required to be protected from outside disturbance of magnetic field with using a simple construction of the device. That is because the magnetic impedance device has high sensitivity so that the device is easily affected by the outside disturbance of magnetic field. Therefore, a current sensor having a magnetic impedance device according to a prior art, for example, includes a magnetic shield and a pair of reverse wound coil for reducing the outside disturbance. This type of current sensor is disclosed in Japanese Patent Application Publication No. 2001-116773. However, this current sensor has a complicated construction so that a manufacturing cost is increased.